A typical form of internal-gear pump comprises a casing and a rotating unit or assembly which is accommodated therein and which comprises a rotatably mounted pinion and an internally toothed annular gear which meshes with the pinion. In such a machine the annular gear can be rotatably supported directly in a bore in the casing or in a bearing ring arranged in the bore. In other design configurations of an internal-gear machine, the annular gear is accommodated in the casing rotatably together with a running ring. A suction chamber and a pressure chamber are respectively formed between the tooth arrangements of the pinion and the annular gear, in which respect in particular the hydraulic pressure of the conveying medium, in the case of a pump, or operating medium, in the case of a motor, which prevails in the pressure chamber, determines the specific bearing loading obtaining between the outer peripheral surface of the annular gear and the bearing surface that co-operates therewith. That applies irrespective of whether the pressure chamber is respectively delimited from the suction chamber by a filling member, or whether the pressure chamber is defined by the teeth of the pinion on the one hand fully engaging into the gaps between the teeth of the annular gear and on the other hand, at a position in approximately diametrally opposite relationship with the region of engagement into the gaps between the teeth, they provide for a sealing contact with the tips of the teeth of the annular gear.
The annular gear is carried at its outer peripheral surface either in a plain bearing, in which case it is possible to set hydrostatic or hydrodynamic bearing conditions, or in a rolling bearing assembly. The permissible conveying or operating pressure of the internal-gear machine is determined by the load-bearing capability of the bearing arrangement. In the case of internal-gear machines with a low conveyor through-put, that is to say which operate at a low speed of rotation and/or which involve a tooth configuration of a small width, the conveying or operating pressure is frequently undesirably limited by the load-bearing capability of the annular gear bearing arrangement so that, in relation to the speed of rotation and/or the width of the tooth configuration, it is possibly necessary in structural terms to arrive at a compromise in order to be able to implement the required conveying or operating pressure, in a practical context.